Adaptive data rate control for narrowcast networks

ABSTRACT

A system to provide narrowcast communications uses adaptive data rate control to individual subscribers such that the effects of precipitation or other link conditions, which are not common to all subscribers, is mitigated. The invention takes advantage of the fact that the narrowcast data consist of packets which are individually addressed to specific subscribers, or groups of subscribers. The narrowcast data is communicated on a plurality of channels, each of potentially differing data rates. The subscribers are assigned a particular channel, based upon their link quality, to receive packets addressed to them. The lower data rate channel will be less affected by adverse link conditions and are hence assigned to subscribers most likely to incur adverse link conditions.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/744,557 filed May 4, 2007 which is a continuation of U.S. patentapplication Ser. No. 09/519,155 filed Mar. 6, 2000, now U.S. Pat. No.7,215,650 issued May 8, 2007, which claims the benefit of U.S.Provisional Application No. 60/149,615, filed Aug. 16, 1999, which areeach herein incorporated by reference for all purposes.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH OR DEVELOPMENT

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REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAMLISTING APPENDIX SUBMITTED ON A COMPACT DISK

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BACKGROUND OF THE INVENTION

The present invention relates to narrowcast communication systems, andmore particularly to a method for providing adaptive data rate controlfor narrowcast communication systems.

A satellite communications narrowcast network typically consists of alarge Earth terminal, referred to as a hub, transmitting an uplink to asatellite repeater. The satellite repeater receives, amplifies, andre-transmits the signal on a downlink to a plurality of subscriberterminals. Each of the subscriber terminals receives the downlink signalfrom the satellite. In many applications the data is individuallyaddressed to a single subscriber terminal or to small group ofsubscriber terminals (narrowcast) which is a subset of all terminalswhich receive the transmission. In a typical application, the hub timedivision multiplexes (TDM) the individually addressed packets data intoa single stream. Each subscriber terminal receives and demodulates thedownlink data stream, but only processes the data which is individuallyaddressed to that particular subscriber terminal.

To service the largest possible number of subscriber terminals, the hubterminal should transmit the TDM data stream at the highest possibledata rate. The received carrier to noise spectral density, C/No, foreach of the subscriber terminals may be different and may be timevarying due to different link and propagation conditions. Such linkconditions are due to, but not limited to, variations in the satelliteEIRP to specific subscriber terminals based upon location, as shown bythe satellite transmit antenna contours in FIG. 2, or differences in theG/T of the subscriber terminals. Propagation conditions typically resultin additional path loss due to precipitation or other atmosphericconditions. Loss due to precipitation, commonly known as “rain fading,”is a frequent occurrence in commercial satellite communications and isespecially prevalent at Ku band (12-18 GHz) and Ka Band (27-40) GHztransmissions. Since the data must be received by all subscriberterminals, at virtually all times, the data rate, modulation, andforward Error Correction (FEC) coding selected for use must be basedupon the worst anticipated C/No among all subscriber terminals in thenetwork. This approach results in the selection of a much lower datarate than most subscriber terminals could support for a majority of thetime.

Several techniques have been developed to overcome the problem of rainfading. For example, U.S. Pat. No. 4,941,199 describes a methodology foradjustment of the Hub EIRP as to maintain a constant C/No as seen by thesatellite. The adjustment is determined by having the Hub terminalmonitor the downlink of its own transmission. This technique is usefulin compensating for rain fading on uplink transmissions but does notprovide any compensation for downlink transmissions.

A technique is described in U.S. Pat. No. 4,228,538 that provides uplinkand downlink rain fade mitigation on point-to-point satellite links.This approach uses feedback from the receiving terminal on the receiversignal quality. The transmitting terminal adjusts its output power inaccordance with the signal quality indication determined by thereceiving terminal. The effectiveness of this approach is limited in thecase of downlink rain fade because, in many cases, the hub terminaluplink EIRP cannot be arbitrarily increased without having an adverseeffect on the transponder operation point or input backoff. Similarschemes have been devised to get around this limitation by using thesignal quality indication to vary the code rate of the transmission.Although this eliminates the aforementioned problem, it is still not anacceptable approach for a narrowcast system, where multiple subscriberterminals will, in general, indicate different signal qualities. Inaddition, adaptation of the code rate alone, from rate=½ to rate=⅞, cantypically only provide about 2 dB of adjustment in the Eb/Norequirement. If one maintains the symbol rate constant, the informationrate will change by 2.4 dB from a rate=½ to rate=⅞ code. The totaladjustment range in the C/No requirement is thus about 4.4 dB. For highlink availability, this is not sufficient for many rain regionsespecially in the higher frequency bands, such as the Ka band.

An applicable technique is found in U.S. Pat. No. 4,837,786. In thispatent, Gurantz and Wright describe a method where two orthogonal BPSKcarriers are provided using QPSK modulation. One BPSK carrier contains ahigh data rate where the other uses a lower data rate. In an alternateembodiment of the referenced invention, individual addressing ofportions of the frame is described. This embodiment is suitable fornarrowcast applications where most ground stations would receive theirdata on the higher data rate channel. Ground stations adversely affectedby precipitation would receive their data on the lower data ratechannel. The limitation of this method is that subscriber terminals onlyhave two choices for their quality of service, mainly the higher datarate or the lower data rate. Moreover, 50 percent of the channelresources have to be dedicated to the lower channel rate.

An opportunity exists for a method of data rate control that: i)compensates for the differences in downlink conditions for a narrowcastnetwork; ii) provides a large range of adjustment for the C/Norequirement; and iii) provides many different data rates or C/Nooptions.

SUMMARY OF THE INVENTION

The present invention comprises a method for providing adaptive datarate control for satellite narrowcast transmissions. The presentinvention enables the data rate to be different to different subscriberterminals thus allowing individualized compensation for the affects ofdownlink rain fading and link parameter variation.

The present invention comprises a satellite repeater to provideconnection between the hub terminal uplink and the subscriber terminaldownlinks. The hub terminal transmits individually addressed packets toa plurality of subscriber terminals on a plurality of channels (FDM orTDM). Subscriber terminals are assigned specific carriers to receiveinformation that will be individually addressed to them. The subscriberterminals shall only need to receive and demodulate one of the multiplechannels originating from the hub terminal's transmission. Thesubscriber terminals are assigned a channel based upon their receivedC/No. Subscriber terminals of like or near like C/No are assigned thesame channel.

In one particular embodiment of the present invention, the number of FDMcarriers and the attributes of each carrier are determined a prioribased upon an initial estimate of C/No metrics within the anticipatedsubscriber population. Attributes of the FDM carriers includecharacteristics such as data rates, the modulation scheme and a codingscheme used on each of the carriers, and the fraction of transponderresources, power and bandwidth, used by each carrier. The estimation ofC/No within the subscriber population is mostly a function of subscriberterminal sizes (G/T), satellite EIRP variation over the location of thecoverage area, and rain fade statistics for the region of service.

Each subscriber terminal determines a signal quality metric such as C/Nobased on the quality of its received signals. The quality metrics arecommunicated to the hub terminal by each of the subscriber terminals.The hub terminal reassigns subscriber terminals to carriers based on thereceived quality metrics. In this way, each subscriber terminalmaintains a maximum data rate as environmental conditions vary over timeby being periodically reassigned to a different carrier in response tothe changing conditions. The newly assigned carrier would then be usedby the subscriber terminal to receive its data services. Thus, whendownlink rain fading occurs, a particular subscriber terminal that wouldbe adversely affected by the rain fade, would be assigned to a differentcarrier that employs a lower data rate, and hence has a lower C/Norequirement.

A return channel is provided for communicating the signal qualitymetrics from the subscriber terminal to the hub terminal. In oneembodiment, the return channel uses the existing satellite communicationlinks. This has the advantage of utilizing existing circuitry toimplement the return channel. Alternatively, the return channel can be aground-based communication link.

In an alternate embodiment of the present invention, the attributes ofeach channel may be dynamically altered to accommodate changes in thedistribution of C/No among the active subscriber terminals. Instead ofproviding a set of pre-defined carriers, the attributes of the eachchannel are periodically redefined to optimize network performancemetrics based upon the received signal quality metrics of the subscriberterminals. Attributes of the channels which can be redefined include,but are not limited to, characteristics such as data rates, themodulation scheme, and a coding scheme used on each of the carriers.Thus, under clear sky conditions, each subscriber terminal can beassigned a particular channel to receive their information based uponthe data rate that each terminal can support and the current loadingtraffic conditions on each of the carriers. As rain fading in any partof the coverage area starts to occur, the assignment of individualsubscriber to channels, as well as the attributes of the individualcarriers may be altered to keep the composite network performanceoptimized.

The present invention is advantageous in that it improves the downlinkdata rate without requiring an increase in satellite resources, such asbandwidth and power, nor does it require additional EIRP to be providedat the hub terminal.

The invention will be better understood upon reference to the followingdetailed description with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pictorial diagram of hub terminal providing a narrowcasttransmission to a plurality of subscriber terminals with rain fadeconditions occurring over some but not all subscriber terminals.

FIG. 2 is a diagram showing contours of differing clear sky linkconditions due to the antenna contours of the satellite transmitantenna.

FIG. 3 shows multiple FDM carriers used to convey the narrowcastinformation.

FIG. 4 is a simplified block diagram of the hub terminal illustratingthe ability to allocate differing transmit powers to different FDMcarriers.

FIG. 5 is an example grouping of the subscriber terminals and the FDMcarrier attributes associated with each group.

FIG. 6 is a simplified block diagram of a subscriber terminal showingthe processing elements required to provide the link quality feedback tothe hub terminal.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

FIG. 1 shows a satellite communications network set up for narrowcastfrom a hub transmitter 100, through an satellite repeater 102, and downto plurality of subscriber terminals 104. Although only four terminalsare shown, the network could consist of many more terminals. The hubterminal 100 transmits a transmission which shall be at least partiallydemodulated by all subscriber terminals in the network. Rain fading 112is present on the downlinks to some, but not all, of the subscriberterminals. It is not uncommon for rain fades to be more than 3 dB at Kuband for many rain regions for world. Furthermore, antenna gain contourscorresponding to the transmit antenna on the satellite repeater 102cause the received downlink C/No to vary amongst subscriber terminals.Such antenna gain contours are illustrated in FIG. 2. Subscriberslocated at or near the −3 dB contour will see a downlink C/No which is 3dB lower than subscriber terminals located near the center of spotcoverage. To ensure service to all subscribers, where service implies aresulting C/No at the subscriber terminal such that the BER is lowerthan the threshold deemed acceptable, a data rate must be selectedconsistent with the minimum expected C/No. As an example, using the −3dB antenna contour with a 3 dB rain fade results in 6 dB variation indownlink C/No. To ensure service to all subscriber terminals within the−3 dB contour and at all times when the rain fade is less than 3 dB, thedata rate must be selected based upon a C/No which is 6 dB lower thanthe C/No of many of the subscriber terminals. This inefficiency resultsin a reduction of system capacity.

In one embodiment of the invention, the system capacity is increased bythe use of multiple FDM (frequency division multiplexed) carriers toconvey the information. It is noted that the invention is not limited toany one type of transmission scheme. The invention is described in termsof FDM signaling by way of example only.

Continuing, FIG. 3 shows a signal configuration comprising six carriers.For discussion purposes, suppose the narrowcast traffic is broken upinto N carriers, numbered 1, 2, . . . N. Further, suppose the narrowcastnetwork occupies transponder resources consisting of a total transponderbandwidth of W Hz and a total transponder transmit power output of PWatts. The n^(th) FDM carrier is allocated a fraction of the totaltransponder bandwidth equal to α_(n)W Hz and a fraction of the totaltransponder power equal to α_(n)P Watts. The parameters α_(n) are chosenunder the constraint that all α_(n) must sum to unity. Those skilled inthe art will recognize that the fraction of the transponder power givento individual FDM carriers is easily adjusted by setting the transmitpower of the individual carriers, relative to one another, at the hubterminal.

FIG. 4 is a simplified block diagram of the hub terminal apparatus usedto provide the plurality of FDM carriers, each with individualizedtransmit power setting. The apparatus contains a data source 200 whichgenerates packetized data 224 that is individually addressed as well asthe specific addresses 226 of the subscriber for which the data packetor packets are intended. A demultiplexer 202 is used to switch the datainto one of the N FDM carrier modulation circuits 204. The flexiblemodulators 204 provide the modulation and FEC coding for each of the Ncarriers. Each modulator is programmable to afford one the best possiblechoice of data rate and coding for each of the FDM carriers. Aftermodulation, each of the FDM carriers is adjusted in signal level by thegain control devices 206. Although only 2 modulators and gain controldevices are pictured in FIG. 4, it should be understood that there isone modulator and gain control device for each FDM carrier. The summingdevice 208 provides a means to sum the FDM carriers into one electronicsignal. The RF circuitry 210 provides upconversion to RF and high poweramplification before the signal is presented to the antenna 212 fortransmission. Preferably, RF circuitry 210 includes an uplink powercontrol system to mitigate the effects of varying weather conditions onthe uplink path. The uplink power control system varies the transmittedpower of the hub to maintain a desired C/No at the satellite.

The hub terminal must also receive transmissions from each of thesubscriber terminals. These transmissions consist of subscriber terminaldata as well as received signal quality metrics such as received C/Noestimates. The subscriber terminal's transmissions are received at thehub terminal by the antenna 212. The RF circuitry 210 provides low noiseamplification and downconversion. The signal is demodulated within thedemodulator 216 which provides data detection and FEC decoding. Thedemodulated bits are then split into data bits 220 and signal qualitymetric bits 222 by the demultiplexer 218. The FDM carrier control 214provides the control signals for assignment of data packets to specificFDM carrier, and FDM carrier attributes such as data rate, code rate,and power level. The basis for control is the received signal qualitymetrics 222 from each of the subscriber terminals.

The hub terminal communicates the assignment of the subscribersterminals to one of the FDM carriers via a forward control channel. Thiscontrol channel contains assignment information and is multiplexed inwith the data services that the subscriber terminal receives. Themultiplexing technique is most logically a time division multiplexed(TDM) approach but could any of a number of other known techniques suchas frequency division multiplexing (FDM), or code division multiplexing(CDM).

Subscriber terminals are categorized into one of N groups, according toan initial signal quality metric. For example, in a preferred embodimentof the invention the current downlink C/No measurement of eachsubscriber terminal is used. Each group of subscriber terminals isassigned one particular FDM carrier for subscribers in that group toreceive their individually addressed packets of information. FDMcarriers are assigned to each of the groups of subscriber terminalsbased upon the minimum C/No of a subscriber terminal within a group. Theattributes of that carrier such as its data rate, modulation scheme,coding scheme, and fraction of the network resources (α_(n)) are setsuch that all subscriber terminals within that group can demodulate theFDM carrier with an acceptable level of data errors, as determined bythe Bit Error Rate for example. An example of the grouping for N=4carriers is shown in FIG. 5. This example provides service forsubscriber terminals with up to 6 dB of variation in their received C/Nowhile still affording subscriber terminals with largest data ratepossible.

Subscriber terminals continually monitor their own signal quality anddetect changes in their received C/No due to rain fading or other timevarying phenomena. These subsequent C/No's, or other link qualitymetrics such as, but not necessarily limited to, bit error rate, arereported back to the hub terminal via a return channel. In a preferredembodiment of the invention, the return channel is shared with thesatellite communications link. The information can be conveyed by any ofa number known multiple access techniques such as time divisionmultiplexing (TDMA), frequency division multiplexing (FDMA), or codedivision multiplexing (CDMA). Of course, the return channel can beprovided by a communication medium other than the satellite link. Forexample, a land-based medium can be used, such as a land line, aline-of-sight wireless link, and the like.

An example of the apparatus used by the subscriber terminal to determineits received C/No and report this value back to the hub terminal isillustrated by the simplified block diagram of FIG. 6. Reception of thesignal transmitted from the hub is accomplished by the antenna 300, theRF equipment 302, which provides low noise amplification anddownconversion, the demodulator 304 and the FEC decoder 306. Theestimation of the Eb/No is provided using information supplied by thedemodulator 304 and/or the FEC decoder 306 using one or more of severalcommonly known techniques. Particularly effective are techniques whichestimate Eb/No based upon decoder bit error correction statistics orblock error detection statistics. Given the estimate of the Eb/No, theC/No can be easily computed by knowledge of the information data rate.This computation is performed in 308. It is frequently desired to add asmall margin of error into the C/No calculation. This is performed bysubtracting 312 a small quantity, in decibels, from the C/No estimate.The purpose of such a margin is to ensure a particular quality ofservice during dynamic C/No conditions using the feedback control systemwith has a non-zero response time. Preferably, the C/No data ismultiplexed 316 into the return channel data stream and transmitted backto the hub terminal via the satellite 102 using the subscriberterminal's modulator 314, the RF equipment 302, and the antenna 300.This has the advantage of using existing hardware to provide a returnchannel. However, the return channel can be provided by a land line, orby a line-of-sight wireless link, or the like.

The subscriber terminal receivers carrier assignments and re-assignmentsthrough the forward control channel. This is typically control data timedivision multiplexed (TDM) in with the service data. In response to acarrier re-assignment message in the forward control channel, thesubscriber terminal re-tunes its RF equipment 302 to the new FDM carrierfrequency.

In alternate embodiments, functions such as the system marginsubtraction or the C/No estimation could be performed at the hubterminal. In such an embodiment, the subscriber terminal would transmitsignal quality metrics, such as, but not limited to, Eb/No, bit errorrate or block error rate, to the hub terminal. In this embodiment, theFDM carrier control 214 would be suitably configured to produce the C/Noquality metric upon which carrier reassignment is then based.

The hub terminal uses the feedback from all subscriber terminals to makechanges in the assignments of subscriber terminals to particular FDMcarriers. The assignment information is conveyed over the forwardcontrol channel. The subscriber terminals then adjust their receivercircuitry accordingly in order to receive subsequent data over the newFDM carrier, as discussed above.

In another embodiment of the present invention, the hub terminal canspecify different FDM carrier attributes instead of making newassignments of subscriber terminal to pre-defined FDM carriers.Attributes of an FDM carrier include characteristics such as data rates,the modulation and coding schemes, and the fraction of transponderresources such as power and bandwidth. In this embodiment, the hubterminal can redefine the attributes of the carriers and assign theredefined carriers to the subscriber terminals. This approach obviatesthe need for defining an a priori set of carriers.

Variations which combine both approaches are contemplated. For example,an initial set of carriers can be defined and assigned to the subscriberterminals. As the downlink conditions change, the hub terminal canredefine the attributes accordingly and make new assignments of thesubscribers to the carriers. In general, the idea is to monitor thedownlink signal quality of a subscriber terminal and change its carrierassignment in order to provide reliable data transfer at a high datarate.

In the foregoing embodiment of the invention, the satellite uses asingle transponder circuit to transmit a plurality of FDM carriers; i.e.all of the carriers are transmitted in a signal occupying a singlefrequency range. In yet another embodiment of the invention, each FDMcarrier could occupy an entire transponder. Thus, for each FDM carrier,the satellite repeater includes a transponder circuit to transmit thecarrier. Each carrier, therefore, occupies a different frequency range.

In this embodiment, the apparatus of FIGS. 4 and 6 can still be used.However, the setting of the electronic attenuators 206 in FIG. 4 wouldbe determined by the characteristics of each of the transponders, suchas the saturated output power of the transponder and gain of thetransponder, as well as the desired transponder operating point, such asthe output power backoff.

Rain fades on the hub uplink will also affect the C/No as measured oneach of the subscriber terminal downlinks. The response of the presentinvention to a 3 dB rain fade on the hub terminal uplink will be for allsubscriber terminals to report a reduction in their measured C/No of 3dB. The assignment algorithms resident in the hub terminal control willreassign subscriber terminals to FDM carriers that require 3 dB lessC/No. Alternative embodiments of the present invention might also adjustcarrier attributes and the number of carriers in response to this fadeevent. To prevent the loss in C/No, as measured by the subscriberterminals due to uplink rain fades at the hub terminal, the presentinvention can be used in conjunction with an uplink power controlsystem. In FIG. 4, the RF circuitry 210 would be provisioned with anuplink power control circuit. The circuit would vary the power level ofthe transmitted signal in response to indications of uplink rain fadeconditions. Such indications can usually be generated within the hubterminal itself by monitoring the satellite downlink.

Prior art techniques, such as the one described in U.S. Pat. No.4,941,199, attempt to maintain the C/No constant at the satellite. Thisis accomplished by providing compensation for uplink fades at the hubterminal. The present invention includes an uplink power control circuitin order to compensate for uplink fades. However, unlike the prior art,the present invention is advantageous in providing compensation forconditions which result in downlink fades at the subscriber terminals.Thus, the present invention is overall less susceptible to the signaldegrading effects of environmental phenomena such as rain fade.

1. In a narrowcast satellite communication network having a hub stationtransmitting to a satellite repeater and a plurality of receivingsubscriber terminals, a method comprising: transmitting data from saidhub station addressed to selected ones of said receiving subscriberterminals to said satellite repeater on a selection of channels;receiving and relaying at said satellite repeater, said data on saidselection of channels to a selected subset of said receiving subscriberterminals using less than all available signal channels; at eachreceiving subscriber terminal, determining signal quality metrics on thebasis of qualities of said signal channels received from said satelliterepeater and addressed to said receiving subscriber terminal;communicating said signal quality metrics determined at said receivingsubscriber terminals to said hub station; at said hub station, groupingpluralities of said receiving subscriber terminals into subgroupsaccording to said signal quality metrics; varying at least one of afrequency bandwidth, a power level, a coding scheme, a modulation schemeand a data rate to produce a plurality of redefined channels; signalingsaid subgroups to use at least one of said redefined channels forsubsequent reception of said data; and narrowcasting from said hubstation said subsequent data to said subgroups of receiving subscriberterminal using said at least one of said redefined channels via saidsatellite repeater.